The process of making float glass is known in the art. For example, see U.S. Pat. Nos. 3,954,432, 3,083,551, 3,220,816, 7,743,630, 8,677,782, 9,016,094, and 5,214,008, the disclosures of all of which are hereby incorporated herein by reference. Generally speaking, in a float glass-making line, batch materials are heated in a furnace or melter to form a glass melt. The glass melt is poured onto a bath of molten material such as tin (tin bath) and is then continuously cooled to form a float glass ribbon. The float glass ribbon is then forwarded to an annealing lehr for further processing and then may be cut to form solid glass articles, such as flat glass sheets. For float glass, the glass batch often includes soda, lime and silica to form soda-lime-silica based flat glass.
Float glass is widely used for windows in commercial and residential buildings, glass furniture, shower doors, and automotive windshields. For many products, float glass must be thermally tempered (undergo heating to at least 580 degrees C., followed by a rapid cooling) to ensure safety in case of breakage. Impurities from raw materials, sulfur from additive(s), and/or contaminations from the float process occasionally and unpredictably form unwanted chemical compounds (e.g., inclusions) during glass formation, which are undesirable defects in the glass. Nickel, for example, is known to spontaneously bond with sulfur to form inclusions of or based on nickel sulfide (of any suitable stoichiometry such as NiS).
Although typically harmless in annealed glass (e.g., made via the float process without any additional heat treatment such as thermal tempering), NiS inclusions are known for causing spontaneous breakage of thermally tempered glass. Moreover, NiS inclusions/defects in thermally tempered glass have caused catastrophic glass failure over long periods of time in installed products. Rejecting defective annealed glass, therefore, serves at least two purposes: a) increase production yield during the expensive thermal tempering and heat soaking stages, and b) minimize catastrophic failures of glass in installed products.
Nickel sulfide exists in different phases at different temperatures. For instance, two specific phases of NiS known are the alpha-phase and the beta-phase. At temperatures below 715 degrees F. (379 C), nickel sulfide is relatively stable in the beta-phase form. Above this temperature, it is stable in the alpha-phase. Therefore, when glass is produced in a furnace, it is likely that any NiS inclusions will be in the alpha-phase. In typical annealed glass, the slow cooling process provided by the annealing lehr allows the NiS ample time to transform to its beta-phase as the glass cools. However, in the fast cooling process used in both heat-strengthened and tempered glass, there is often insufficient time to complete the phase transition (which is a relatively slow process). The NiS inclusions are therefore trapped in the glass in their high-temperature alpha-phase. However, once the glass cools past the phase change temperature, the NiS inclusion seeks to reenter the lower energy beta-phase. For trapped inclusions, this process takes anywhere from months to years. This may have no effect on glass, were it not for the point that when the NiS changes from alpha-phase to beta-phase, it increases in volume such as by 2-4%. This expansion may create localized tensile stresses which can lead to glass failures.
Nickel sulfide is a compound that comes in various forms as well. The most common forms of nickel sulfide are Ni7S6, NiS, NiS1.03, Ni3S2 and Ni3S2+Ni. When viewed under an electron microscope, Ni7S6, NiS, and NiS1.03 are yellow-gold in color and have a rugged surface similar to a golf ball. These three types are non-magnetic and have been found to cause failure in tempered glass.
Various methods have been used for inline detection of NiS inclusions and other micro-defects of similar size scale (e.g., 50-150 microns sized defects). U.S. Pat. No. 7,511,807, incorporated herein by reference, for example directs light at the glass and looks for light scattering in order to detect inclusions. The detection cross-section in such a manner, however, is small around the same as the defect size. Conventional techniques for detecting inclusions therefore have been inefficient and sometimes ineffective.
In view of the above, it will be apparent that there exists a need in the art for an improved method of making glass, and controlling glass quality, including an improved method and/or apparatus for detecting inclusions in soda-lime-silica based glass.